When building radio frequency (RF) systems that involve multiple antennas, such as multiple input/multiple output (MIMO) or active antenna systems (AAS), a critical design constraint is that the local oscillator (LO) clocks attached to the RF circuits attached to each antenna are synchronized in phase. This phase synchronization of the LOs allows, through constructive and destructive techniques, the resulting RF signals to be processed spatially. This spatial processing may allow for redundancy, increased effective bandwidth, optimized coverage through beam forming, and some combination thereof.
If the LOs are not synchronized in phase, spatial processing of the RF signals is still possible where the delays (or phases) of the LOs associated with each antenna are calibrated. The calibration of the antennas may be costly, either in time or in equipment, especially where the possible delay (or phase) of each LO can effectively be completely arbitrary. In addition, the LO phases drift with respect to each LO over time and in response to environmental conditions (such as voltage, temperature, humidity, and the like) may limit the usefulness of any initial phase calibration and may mandate that additional techniques, such as periodic calibration or background calibration, be performed.
It is, therefore, desirable to provide a system and method for synchronizing local oscillators.